Negative pressure relief valves of this type, which are located within the fuel system, move into the closed position and seal the fuel system against the exterior when there is a negative pressure within the fuel system. A negative pressure arises especially during standing phases of the vehicle since the fuel system and the fuel contained in it cool off. If the negative pressure relief valve also remains in the open position over a longer time interval, the presence of a leak within the fuel system can be deduced. When the fuel system is intact, the existing negative pressure, however, acts on it over a comparatively long time interval so that the negative pressure switch remains in the closed position for a correspondingly long time interval. In this instance, it can happen that the negative pressure switch locks in the closed position and no longer moves into the open position even when the pressure rises. Since a blocked negative pressure switch is no longer suitable for the detection of leaks, its mobility must be ensured by regular testing. A process which is suitable for this purpose and a corresponding device can be taken as known from U.S. Pat. No. 6,327,901 B1. The negative pressure within the fuel system there is first reduced by operating a valve and then it is checked whether the negative pressure switch has moved into the open position. When the negative pressure switch successfully moves into the open position within a defined time interval, the valve is closed again and the fuel system is exposed to a negative pressure. Then it is checked whether the negative pressure relief valve is moving into the closed position within a further time interval. If the negative pressure switch successfully passes all tests, this is evaluated as an indication of its mobility.
The circumstance that the process and the device intended for its implementation are made comparatively expensive and complex and thus lead to increased production costs can be regarded as disadvantageous.
The object of this invention is therefore to devise a process and a device for testing the mobility of the negative pressure relief valve of the fuel system of a motor vehicle which can be implemented more easily and economically.